Ageing female rats subjected to a standard lighting schedule (L:D = 14:10) frequently enter a state of persistent vaginal cornification (PE) in which follicular development occurs without ovulation. The function of the pituitary/hypothalamic complex in these animals was compared with that of young cyclic rats by measuring levels of circulating luteinizing (LH) and follicle stimulating hormone (FSH) by radioimmunoassay after ovariectomy and after treatment with gonadal steroids. Plasma LH levels were similar in old PE rats to those of young cyclic animals on the morning of pro-oestrus. Both the post-ovariectomy increase of LH and the oestrogen-progesterone evoked surge of LH were significantly reduced in the PE group. However, the plasma FSH concentrations were higher in PE rats than cyclic controls at pro-oestrus and they attained similar values after ovariectomy in young and old groups. These results indicated that the capacity of the pituitary gland of ageing PE rats to secrete LH was impaired whereas FSH secretion was not reduced.